Aluminum alloy pipes have recently been used in the piping of refrigerant circuits of automotive air conditioners to reduce the weight of automotive bodies; however, vibration in the compressor or the like may cause the piping to resonate, resulting in noise generation. In order to damp the piping resonance, composite hoses formed of multiple layers of rubber and resin have thus been incorporated in the middle of the piping.
In the meanwhile, HFC134a has been used in place of chlorofluorocarbons, which are ozone-destroying substances, as a refrigerant for automotive air conditioners. HFC134a has zero ozone depletion potential but has high global warming potential to accelerate global warming. Thus, as an alternative to HFC134a, the natural refrigerant CO2 is recommended because of low global warming potential.
However, in the use of the CO2 refrigerant, the refrigerant circuit piping needs to withstand a temperature range of 140° C. to 180° C. and a pressure range of 13 to 15 MPa, as compared with 120° C. to 140° C. and 1.7 to 1.8 MPa in the use of the HFC134a refrigerant.
Instead of known composite hoses formed of the multiple layers of rubber and resin, which cannot withstand such high temperature and high pressure, a vibration-absorbing hose having a stainless-steel bellows has been developed.
The stainless-steel vibration-absorbing hose has a metal hose wall and has thus excellent gas permeability compared with known composite hoses formed of rubber and resins; hence, there is no leakage of the refrigerant therefrom. Thus, the stainless-steel vibration-absorbing hose is used for not only CO2 refrigerant but also the current HFC134a refrigerant and the like in order to reduce the amount of leakage of the refrigerant to the exterior to zero.
However, when the vibration-absorbing hose is incorporated in the refrigerant circuit, the following problems arise: in the present circumstances, the bellows of the vibration-absorbing hose must be composed of stainless steel in view of processability and strength. On the other hand, the refrigerant circuit piping needs to be composed of aluminum (or an aluminum alloy) in view of a reduction in the weight of an automotive body and costs; hence, it is difficult to change the material from aluminum to stainless steel. Thus, the stainless-steel vibration-absorbing hose needs to be connected with the aluminum piping. However, it is significantly difficult to achieve a reliable joint with high strength and high hermeticity by only mechanically fitting or screwing these metal pipes. Furthermore, joining aluminum with stainless-steel by welding or brazing easily forms a brittle intermetallic compound in the joint; hence, also in this case, it is significantly difficult to achieve a reliable joint with high strength and high hermeticity.
As a method for joining a steel material to aluminum, a method including roughening a surface of a base material composed of a steel material to form irregularities, temporarily forming an aluminum layer, and forming a diffusion layer composed of an Fe—Al intermetallic compound by radio-frequency heating while pressing the aluminum layer from the surface side is disclosed (e.g., see Patent Document 2).
However, the method aims to improve the abrasive resistance and smoothness of the surface of the base material by forming a diffusion layer composed of the intermetallic compound. As long as the intermetallic compound is formed, a reliable joint with high strength and high hermeticity is not obtained.
To overcome the foregoing problems, the inventors have developed a joint structure for connecting dissimilar metal tubes disclosed in Japanese Patent Application No. 2004-135884.
As shown in FIG. 4, the joint structure according to this invention is obtained by crimping joining ends of a metal bellows tube A composed of stainless steel or the like and a metal tube B composed of a material, such as aluminum, different from that of the metal bellows pipe with a specific thermosetting resin R and then performing fixation by heat. Specifically, a straight tube portion (S) located at an end of the metal bellows tube A provided with a bellows (Q) is brazed to the inner surface of the base of a nipple (U). A reinforcement portion (W) is disposed between the outer surface of the base and a crimp collar (V) and fixed by crimping. The head (Y) of a socket (X) is fixed outside the nipple (U) by crimping. The thermosetting resin R is fed into the socket (X). An end of the metal tube B is inserted into a gap between the socket (X) and an end of the nipple (S) and fixed by crimping from the outside of the socket (X). The thermosetting resin is cured by heating. Thereby, the metal bellows tube A is joined to the metal tube B by fixing their joining ends.
As described above, it is possible to easily produce the joint structure for connecting dissimilar metal tubes, the structure having excellent strength and hermeticity.
The inventors successfully commercialized the joint structure on the basis of the prior invention. From the results of various studies, however, it was found that the joint structure had room for improvement.
That is, as shown in a figure, the joint structure includes a connection section having two crimp regions C1 and C2 and a non-crimped region F provided therebetween. The metal bellows tube A is connected to the metal tube B in the connection section. Lc1, Lc2, Lf, and Lt in parentheses represent lengths of the regions and the section. Lt corresponds to the entire length of the nipple (U). In the case where the non-crimped region F is not provided, in other words, in the case where a structure is used in which the head (Y) of the socket (X) is in contact with the crimp collar (V) and the crimp region C1 is continuous with the crimp region C2 while Lf is zero, the following problems arise: Stress concentrates on the boundary between the crimp regions C1 and C2 to reduce the strength of the nipple (U). Stress due to crimping in the crimp region C2 causes the crimp region C1 that has already been secured to deform or strain to loosen the fixation, leading to the detachment of the reinforcement portion (W) of the metal bellows tube (A) from between the crimp collar (V) and the nipple (U). Thus, the entire length Lt, corresponding to the connection section for connecting tubes A and B, of the nipple (U) needs to be longer by the length Lf (typically, 10 to 20 mm or more) of the non-crimped region F.
In the case where a refrigerant circuit connected to such dissimilar metal tubes is mounted on an automobile or the like, the piping structure needs to be minimized in view of other co-resident apparatuses, piping, and their components. Thus, the entire length Lt of the nipple (U) needs to be minimized. For example, the metal tube B composed of an aluminum alloy typically has a bend near a portion connected to the metal bellows tube A. A space occupied by the piping structure decreases as a bend starting point P is closer to the metal bellows tube A side, resulting in downsizing.
In the joint structure disclosed in the prior invention, a groove (Z) is formed before crimping in order to fix the socket (X) at a predetermined position on the nipple (U). Consequently, a groove-forming process is required, and the thickness of the nipple (U) must be increased by the depth of the groove. This disadvantageously increases production and material costs.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-195474
Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-310161